GEOLOGIC MAP OF THE EL RITO 7.5MINUTE QUADRANGLE, RIO ARRIBA COUNTY, NEW
MEXICO
DESCRIPTIONS AND REPORT
BY
DANIEL J. KONING1, GARY A. SMITH2, AND SCOTT ABY3
Year 2 of 2-Year STATEMAP Quadrangle
May, 2008
1
New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, 801 Leroy Place,
Socorro, NM 87801-4796; dkoning@nmt.edu
.
2
Department of Earth and Planetary Sciences, Northrop Hall, University of New Mexico,
Albququerque, NM 87131-1116
3
Muddy Spring Geology, Box 488, Dixon, NM 87527
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EXECUTIVE SUMMARY
The El Rito quadrangle exhibits geologic features that are particularly useful in regards to Santa
Fe Group stratigraphy (i.e., the layers of sediment filling the Rio Grande rift), Rio Grande riftrelated structures and faulting, and terraces related to erosion of the Española Basin over the past
6 million years. The El Rito quadrangle, in particular the southern quadrangle, offers good
exposures of the Santa Fe Group stratigraphic units filling the Abiquiu platform. We use the
term Abiquiu platform in a structural sense to indicate the northwest arm of the Española Basin,
located northwest of the Santa Clara faults and Embudo fault systems, that approximates a
faulted bench and where basin-fill is approximately 1 km-thick (Baldridge et al., 1994). PreSanta Fe Group strata are only found locally near the northern quadrangle boundary and include
Ortega Quartzite (originally ~1.6 billion year old sandstone that has been highly metamorphosed
into a quartzite) and El Rito Formation (40?-55? Ma conglomerate and sandstone inferred to
have been deposited during the Laramide orogeny -- Logsdon, 1981; Smith et al., 1961). A
quartzite-dominated gravel and sand known as the Ritito Conglomerate (Barker, 1958; Bingler,
1968), which unconformably overlies the El Rito Formation, is probably Oligocene-age and may
or may not relate to earliest rifting. Found along the northern and western quadrangle
boundaries, the Abiquiu Formation constitutes the lower Santa Fe Group in the map area and
achieves thicknesses of 250-350(?) m. It is recognized by its white, tuffaceous sand and felsicdominated volcanic gravels. We place the base of the Tesuque Formation where the daciticandesitic content of the volcaniclastic gravels increases – specifically, where the felsite gravel is
so overwhelmed by these more intermediate clasts that their percentage is less than 10% (at least
in the lower part of the Tesuque Formation). This increase of dacite-andesite gravels relates to
the erosion of a formerly major volcanic edifice, now eroded and buried in the south-central San
Luis Basin (Ingersoll et al., 1990), and appears to coincide with the deposition of orangish,
predominately very fine- to fine-grained sand beds within the volcaniclastic sediment. We apply
the name Plaza lithosome to this dacite- and andesite-bearing volcaniclastic, a slight
modification of the Plaza petrosome of Ingersoll et al. (1990), and suggest future elevation of
these strata to a member-rank term of the Tesuque Formation. The lower and middle parts of the
Plaza lithosome interfinger downstream (southwest) into strata where fine orange sand is
approximately subequal or dominant and intercalated with these same gravels. The latter strata
belong to the Chama-El Rito Member of the Tesuque Formation (Galusha and Blick, 1971; May,
1980 and 1984). Our mapping demonstrates that in the eastern quadrangle one can differentiate
an upper unit of the Plaza lithosome. This upper unit has 10-50% gravel composed of tuff and
rhyolite (similar in composition to those seen in the Abiquiu Formation) in addition to 1-3%
orangish granite clasts and 1-5% intermediate intrusive clasts (probably a granodiorite, tonalite,
or quartz diorite). These intrusive clasts seem to occur slightly lower in the Tesuque Formation
to the west (but still in the middle part of the formation), and were not noted in the lower
Tesuque Formation nor Abiquiu Formation. We have not identified Proterozoic exposures in the
Tusas Mountains that satisfactorily match these granitoid clasts, based on limited
reconnaissance, and it is possible they were recycled from an older, non-preserved gravel deposit
originally derived from erosion of Proterozoic rock.of the Sangre de Cristo Range. The change
in clast composition and paleocurrent directions (from southwest in the lower to middle Plaza
lithosome to southeast in the upper unit of the Plaza lithosome) is consistent with unroofing of
older volcaniclastic sediment from an uplifting Tusas Mountains during a time of eastward, rift-
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related tilting in the middle Miocene. A progressive up-section decrease in bedding dips
measured east of El Rito Creek supports active tectonism and tilting of fault-bounded blocks
during deposition of the Tesuque Formation. This eastward tilting might also account for the
southeastward change in paleocurrent directions, in a scenario where the Ojo Caliente fault
(located 2-3 km to the east in the adjacent Ojo Caliente quadrangle) became increasingly active
in the middle Miocene relative to east-down faults found to the southwest near Abiquiu. Lastly,
the uppermost Santa Fe Group stratigraphic unit preserved on this quadrangle is the Ojo Caliente
Sandstone. This very fine- to medium-grained, quartz-rich sand represents eolian deposition
beginning ca. 13.4 Ma (Koning et al., 2005a).
The El Rito quadrangle encompasses an important structural transition in the Rio Grande
rift between the western San Luis Basin structural terrain (including the Tusas Mountains) and
the Abiquiu platform of the Española Basin. Two west-down faults continue into this
quadrangle from the north. The more prominent of these faults is the Potrero fault (sensu Kelley,
1978). Located on the eastern margin of Arroyo Seco, this fault projects SSE towards CCC
spring. Stratigraphic constraints suggest that near CCC spring this fault intersects a major westdown, northeast-striking fault. Together, these two faults have resulted in the preservation of
stratigraphically higher, less tuffaceous and consolidated, and likely more permeable strata near
Highway 554 that runs south of El Rito. These stratigraphically higher strata would include the
upper unit of the Plaza lithosome, Chama-El Rito Member, and the Ojo Caliente Sandstone of
the Tesuque Formation. East of that fault pair, strata appear to be more tuffaceous, consolidated,
and locally more cemented (i.e., lower to middle Plaza lithosome and the underlying Abiquiu
Formation). Based on these field observations and hydrogeologic observations of the Las
Placitas well (Geohydrology Associates, Inc., 1979), we suggest the possibility that wells drilled
east of the southern Potrero fault may encounter strata with lower groundwater-yields than wells
drilled on the west side of the fault.
Although obscuring Santa Fe Group stratigraphy in much of the western and middle parts
of the quadrangle, Pliocene-Quaternary terrace deposits preserved here are more complete and
extensive than most other places in the Española Basin (with the possible exception of the lower
Rio Chama). These terrace deposits occupy seven major levels, and consist of sandy gravel
derived from the Tusas Mountains. The rounded to subrounded gravel, which includes abundant
cobbles together with boulders and pebbles, is composed of quartzite and felsic-intermediate
volcanic gravel. These terraces can be used to infer the erosion rates of this part of the Española
Basin during the past 3 million years, such has been done using the Rio Ojo Caliente terraces to
the east (Newell et al., 2004), but climate changes also have played a likely role in their
formation (Dethier and Reneau, 1995).
Basalt flows at Sierra Negra, located in the extreme southwest corner of the quadrangle
and dated at 5.56±0.12 Ma Ma (Maldonald and Miggins, 2007), have preserved a high-level
geomorphic surface in the study area. A basalt plug found on the northern slopes of Sierra Negra
is the probable source for the flow capping the mesa immediately east of the summit of Sierra
Negra. The thinner basalt capping the summit does not appear to correlate chemically or
mineralogically with these other flows (Appendix 1). It may be a slightly older basalt flow
capping an older (late Miocene), higher erosion surface, or else possibly represents an exhumed,
middle Miocene basalt flow. The 5.6 Ma basalt capping the eastern mesa has been vertically
offset 11 m (down-to-the-east) by the Madera Cañon fault, indicating that rift-related faulting
continued into the Pliocene, at least locally -- although the highest vertical displacement rates
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along these western rift faults probably peaked in the middle Miocene (Baldridge et al., 1994;
Koning et al., 2007a).
DESCRIPTION OF MAP UNITS
Grain sizes follow the Udden-Wentworth scale for clastic sediments (Udden, 1914; Wentworth,
1922) and are based on field estimates. Pebbles are subdivided as shown in Compton (1985).
The term “clast(s)” refers to the grain size fraction greater than 2 mm in diameter. Clast
percentages are based on counts of about 100 clasts at a given locality, except where noted.
Descriptions of bedding thickness follow Ingram (1954). Colors of sediment are based on visual
comparison of dry samples to the Munsell Soil Color Charts (Munsell Color, 1994). Surficial
units are only delineated on the map if estimated to be at least 0.5 m thick. Sand textures and
compositions described using a hand lense in the field. Soil horizon designations and descriptive
terms follow those of the Soil Survey Staff (1992) and Birkeland (1999). Stages of pedogenic
calcium carbonate morphology follow those of Gile et al. (1966) and Birkeland (1999). Note
that we use “El Rito Creek” rather than the “El Rito” depicted on the published USGS
topographic base map in order to differentiate this topographic feature from the town of same
name.
FIGURE CAPTIONS
Figure 1: Areas mapped by the respective authors.
Figure 2: Map units correlated with respect to time.
Figure 3: Depiction of terrace profiles for the part of El Rito Creek south of its southward bend
by Placitas.
Figure 4: Simplified illustration of stratigraphic relations of the map units.
Figure 5: Paleocurrent data for various stratigraphic units north and east of El Rito Creek
(where exposures allow one to ascertain stratigraphic position). These units include: Abiquiu
and Los Pinos Formations (Ta and Tlpc), the lower Plaza lithosome (Ttpl), middle Plaza
lithosome (Ttpm) in addition to interfingering fine sand intervals (Ttcf), combined lower-middle
Plaza lithosomes (Ttpml), and upper Plaza lithosome and overlying Chama-El Rito Member
(Ttpu and Ttc). Rose diagrams used paleocurrent data as depicted on the geologic map, with
each channel margin weighted as one, a channel-trend weighted as two, and trough crossstratification weighted as one. Clast imbrication data were weighted as one for every 5
measurements (e.g., 15 clast measurements would be weighted as 3).
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QUATERNARY COLLUVIAL, EOLIAN, AND LANDSLIDE DEPOSITS
Qct
Colluvium and talus (Holocene to Pleistocene) – Hillslope mantles of quartzite and
basaltic gravel in a sandy matrix that form downslope of high-level gravle deposits and
on the northern slopes of Sierra Negra. Loose to weakly consolidated and probably 1-2
m-thick.
Qc
Tt
Colluvium over unidentified Tesuque Formation (Holocene to Pleistocene) –
Clayey-sandy gravel typically located on slopes beneath terrace surfaces that obscures
underlying strata. Not described in detail, but position on the footslope strongly suggests
that the material is colluvial. Probably 1-2 m-thick.
Qc
Ta
Colluvium over unidentified Abiquiu Formation (Holocene to Pleistocene) –
Clayey-sandy gravel typically located on slopes beneath terrace surfaces that obscures
underlying strata. Not described in detail, but position on the footslope strongly suggests
that the material is colluvial. Probably 1-2 m-thick.
Qls
Landslide deposits (Pleistocene) – Subsided, deformed and internally faulted blocks of
basalt and underlying Tesuque Formation on the steep slopes of Sierra Negra. Surface is
hummocky. Landslide is typically elongated in the transport direction (latter shown by
arrows on the map). Many landslides have steep head-scarps as tall as 100m. Locally,
the landslide deposits are subdivided by relative age based on inset and cross-cutting
relations as follows:
Qlso – Older landslide deposits
Qlsm – Middle landslide deposits
Qlsy – Younger landslide deposits
These interpreted ages are relative to surrounding landslides and do not imply that the
unit is everywhere the same absolute age. For example, whereas Qlsy is younger than an
adjoining Qlsm landslide, it may not be the same absolute age as a Qlsy on the other side
of Sierra Negra.
Qlsb Landslide deposits consisting of factured basalt flows (Pleistocene) – Basalt flows
similar to unit Tb that have been displaced downward and deformed by mass-wasting
processes. Locally extensively fractured as to leave a dense collection of large boulders
on the surface.
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Qse
Sheetwash and eolian deposits with minor stream-channel alluvium (Holocene to
upper Pleistocene) – Accumulations of fine- to medium-grained sand with variable
amounts of gravel, especially where associated with terrace units. Mostly mapped as a
thin deposit capping the Tg3 surface. In the higher-elevation parts of this surface, the
Qse unit preferentially thickens into shallow channels, although it still consists primarily
of fine- to medium-grained sand. This suggests reworking of primarily eolian materials.
Qse also forms a broad hillslope mantle along the margin of an unnamed drainage south
of Sage Tank. The apparent abundance of fine sand, albeit admixed with large quantities
of quartzite gravel, led us to map this latter area as Qse rather than Qct, but either could
be defended at this location. Unit also locally caps the drainage divide east of El Rito
Creek. Here, it consists of light yellowish brown, clayey-silty (estimate 1-10% fines)
very fine- to very coarse-grained sand (mostly very fine to fine sand) that is massive.
Note that this deposit also covers Pleistocene-age terrace deposits, but we chose not
to depict it in order to better depict the terrace deposits, which we felt were more
important. Sediment is moderately consolidated, non-cemented, and 1-2 m-thick.
QUATERNARY AND PLIOCENE ALLUVIAL DEPOSITS
Qayl Younger alluvium occupying the lower topographic positions on valley floors (i.e.,
modern stream channels and adjoining floodplains) (upper Holocene to modern) –
Generally sandy gravel and gravelly sand. Contains abundant quartzite cobbles and
boulders where drainage basins include Ritito Conglomerate, the upper unit of the Plaza
lithosome, or Pliocene-Pleistocene terrace deposits; otherwise, pebbles are the main
gravel size. Gravel are poorly to moderately sorted. Pebbles are subrounded to
subangular and composed mostly of felsic to intermediate volcanic clasts with minor
quartzite clasts, whereas cobbles and boulders typically consist of subrounded to rounded
quartzite with minor felsic to intermediate volcanic clasts. Sand is very pale brown to
light gray to gray and very fine- to very coarse-grained (mostly medium- to very coarsegrained), subangular to subrounded, poorly sorted, and has a composition consistent with
lithologies present upstream in a given drainage. Bar and swale topography, in addition
to recent stream-related channelization, produce as much as 1-1.5 m of relief. Probably
only 1-2 m-thick in smaller drainages, and 3-5 m-thick under larger drainages. Unit
unconformably overlies thicker sand and gravel interval that is late Pleistocene to
Holocene in age. A cuttings log for a well located approximately 1 km southeast of El
Rito suggests that this older interval is approximately 10 m-thick.
Qayi Younger alluvium occupying intermediate topographic positions on valley floors
(upper Holocene) – Sand, silty sand, and gravel occupying intermediate terrace positions
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on valley floors. Unit is probably a cut-and-fill deposit inset into unit Qayh or a strath
cut into Qayh. 1-3 m-thick.
Qayh Younger alluvium occupying higher topographic positions on valley floors (upper to
middle Holocene) – Sand, silty sand, and gravel occupying the higher terrace positions
on the floor of valleys (below the Qtr and Qtt Pleistocene units). In the headwaters of
Arroyo del Perro this unit is a sheetlike, graded, hillslope mantle of gravelly sand that is a
mixture of sheetwash and eolian deposits similar to Qse. This unit differs from Qse in its
tendency to form a distinct geomorphic surface and to be continuously traceable from the
hillslope position down into fill terraces in the fingered headwater tributaries of Arroyo
del Perro and the main branch of Alamosa Canyon. The alluvial-fill part of the unit is
about 3-m thick and consists of a basal sandy gravel layer about 1 m thick overlain by
orange and brown sands and local clay-silt beds. Soil development seems to be minimal
in the few places where examined in cut-banks. It is possible, that as mapped, this unit is
not time correlative everywhere. Along lower El Rito Creek, this unit forms a thick fill at
least 6 m-thick. Here, it consists of medium to thick, tabular to broadly lenticular beds of
very fine- to medium-grained sand with subordinate coarse to very coarse sand and minor
pebbles. Sand is very pale brown to pink (7.5-10YR 7/3), subangular to subrounded,
moderately sorted, and a volcanic-bearing litharenite to lithic arkose. Minor thin, tabular
beds of silty (estimate 1-25% silt) very fine- to fine-grained sand. Also minor very thinto medium, lenticular beds of clast-supported sandy pebbles with minor cobbles. Surface
of unit typically rises 1-4 m above adjacent active channels. Base of unit lies
unconformably over scoured Tesuque or Abiquiu Formations, except in larger drainages
where it likely overlies an older late Pleistocene-Holocene gravelly sand-sandy gravel.
A cuttings log for a well located approximately 1 km southeast of El Rito suggests that
this older interval is approximately 10 m-thick.
.
Qayu Undivided valley-floor alluvium (Holocene) – Sand, silty sand, and gravel of deposits
Qayh, Qayi, and Qayl that were not individually differentiated.
Qao
Sandy alluvium occupying the highest topographic positions on valley floors (middle
Holocene to upper Pleistocene) – Isolated remnants of sandy gravel that are 1-3 m
higher than adjoining Qayh deposits. Up to approximately 3-5 m-thick.
Qaf
Younger alluvial fan deposits (Holocene to Pleistocene) – Sandy gravel to gravelly
sand to clayey sand alluvium forming fans at the mouths of low-order drainages and
along terrace risers on the west side of El Rito valley. Unit appears to grade with the
Qtr4 terrace tread. Thickness unknown.
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Qgo1 Older gravel alluvial fan deposits (middle Pleistocene) – Sandy gravel to gravelly sand
alluvium forming a broad, east-sloping alluvial fan or piedmont-slope near the
headwaters of Alamosa Canyon. Unit appears to grade with the Qtr1 terrace tread, but
its eastward slope suggests deposition by small tributary streams or slopewash processes
rather than deposition by El Rito Creek. Unit generally less than 5 m-thick.
Qtr
Quaternary terrace deposits associated with El Rito Creek (Pleistocene) – Sandy
gravel terrace deposits along El Rito Creek. The sand is pale brown to light gray (10YR
6/2-3), mostly medium- to very coarse-grained, subrounded, poorly sorted, and has more
volcanic lithic grains compared to pinkish potassium feldspar. Gravel consists of pebbles
and cobbles, with 2-5% boulders, that are clast-supported and imbricated. Boulders and
cobbles are subrounded to rounded and contain >80% quartzite, with the remainder being
rhyolite, dacite, and andesite clasts. Volcanic clasts dominate the pebble fraction in the
lower terraces, but higher terraces have an approximately subequal volcanic : quartzite
ratio and the volcanic fraction is dominated by felsic clasts. In the pebble fraction, there
is also minor mylonitized quartzite and 1-2% granitoid clasts. 1-5 m-thick, except where
noted, with the lower terraces appearing to be thicker than the higher terraces. Terrace
surfaces (treads) are well-preserved , typically extensive, and overlain by 1-2 m of Qse.
Note that Qse was not mapped over Qtr terraces because doing so obscures the
underlying terrace – we think the latter is more important to depict on the map. Six main
levels were recognized that are listed below; note that locally some of these levels are
subdivided based on slight differences of tread (terrace surface) height (e.g., Qtr4a,
Qtr4b, and Qtr4c). The difficulty in correlating these sub-levels contribute to the
irregularity of the profile (Figure 3). Note that the lower terraces (Qtr6 through Qtr3)
diverge slightly in a downstream direction (i.e., about 6 m divergence over 7 km; Figure
3). For each of the terraces listed below, we give their height above El Rito Creek and
their interpreted ages. These terraces can be used to infer the erosion rates of this part of
the Española Basin during the past 3 million years, such has been done using the Rio Ojo
Caliente terraces to the east (Newell et al., 2004), but climate changes also have played a
likely role in their formation (Dethier and Reneau, 1995, p. 292).
Qtr6 – Lower Pleistocene gravel terrace. Unit is more discontinuous than the
aforementioned older units, being found only locally in the southern part of the
quadrangle and near Placitas. Strath is 10-14 m above the modern stream. Probably
correlates to the 15-6 m-high terrace of Dethier and Reneau (1995, fig. 2), which has an
inferred age of 26-44 ka based on radiocarbon dating. Probably correlates to the Qtc7
terrace on the Medanales quadrangle to the south (Koning et al., 2004). Inferred age of
26-40 ka.
Qtr5 – Upper lower Pleistocene gravel terrace. Strath is 12-21 m above the modern
stream (mostly 18 m). Correlates with terrace deposits Qtr4and Qtc6 in the Medanales
quadrangle to the south, which in turn have an inferred age of 40-70 ka (Koning et al.,
2004; Dethier and Reneau, 1995; Dethier and McCoy, 1993); we favor an age closer to
70 ka. This terrace locally is as much as 6 m-thick.
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Qtr4 – Lower middle Pleistocene gravel terrace. Strath is 21-38 m above the modern
stream. Correlates with terrace deposits Qtr3 and Qtc5in the Medanales quadrangle to
the south, which have an inferred age of ca. 130 ka (Koning et al., 2004; Dethier and
Reneau, 1995; Dethier and McCoy, 1993).
Qtr3 – Middle Pleistocene gravel terrace. Strath is 43-61 m above the modern stream.
Correlates wih terrace deposit Qtr1 in the Medanales quadrangle to the south, which in
turn has an interpreted age of 250-400 ka (Koning et al., 2004; Dethier and Reneau, 1995;
Dethier and McCoy, 1993).
Qtr2 – Upper middle Pleistocene gravel terrace. Strath is 73-88 m above the modern
stream. Probably correlates to terrace deposit Qtc4 in the Medanales quadrangle to the
south, which has an inferred age of 250-400 ka based on amino-acid ratios of fossil
gastropods (Dethier and McCoy, 1993).
.
Qtr1 – Upper Pleistocene gravel terrace. Strath is 98-104 m above the modern stream.
The extensive surface of Qog1 grades into this surface. Based on its strath height, this
terrace is inferred to correlate with terrace deposits containg the Lava Creek B Ash (620
ka; Sarna-Wojcicki et al., 1987) along the Rio Chama south of Medanales (Koning et al.,
2004; Dethier et al., 1990; Dethier and Reneau, 1995). Its strath height is similar to
terrace deposits mapped as Qtv5 and Qtoc5 on the La Madera and Ojo Caliente
quadrangles to the northeast and east (Koning et al., 2007b and 2005b). These deposits
were also correlated to the Lava Creek B ash-bearing terrace deposits along the Rio
Chama.
Qtru Quaternary terrace deposit, undifferentiated (middle to upper Pleistocene) – Sandy
gravel deposits deposited by El Rito Creek that were not correlated to one of the six
terraces listed above. Lithologic properties are similar to that of Qtr. 1-5 m-thick.
Qtt
Quaternary terrace deposits associated with tributaries of El Rito Creek (middle to
upper Pleistocene) – Sandy gravel terrace deposits that underlie distinct treads 6-25 m
above modern channel elevations along major tributary drainages to El Rito Creek. We
generally did not notice multiple treads within individual drainages (other than the lower
Qao), suggesting that Qtt may possibly represent a single aggradational/degradational
cycle that is correlative between drainages. No effort was made to trench into these
deposits to describe texture or soil development; description based on surface
characteristics. Correlation between drainages and from place to place within drainage
basins was generally not attempted, except where such terraces obviously grade into one
of the El Rito Creek terrace deposits (Qtr). In these cases the tributary terrace deposits
were numbered to match their equivalent El Rito Creek terrace (as listed below). 1-3 mthick.
Qtt5 – Upper lower Pleistocene gravel terrace. Correlates with terrace deposit Qtr5.
Page 9 of 31
Qtt4 – Lower middle Pleistocene gravel terrace. Correlates with terrace deposit Qtr4.
Qtt3 – Middle Pleistocene gravel terrace. Correlates with terrace deposit Qtr3.
Tg3
Coarse gravel underlying uppermost, extensive high-level geomporphic surfaces
(upper Pliocene) – This unit ranges in thickness from 2 to 8 m thick, suggesting burial
of a dissected landscape by alluvial fill. Its strath stands 190-221 m above the modern El
Rito Creek. The fill consists predominantly of cobble to boulder gravel, with clasts as
large as 1 m across but typically the largest clasts are about 50 cm across. Clast
composition is estimated to be 75-85% gray quartzite, 1-2% white vein quartz, generally
less than 1% granitic and metarhyolitic clasts, and the remainder being volcanic clasts of
widely variable composition, texture, and color. Quartzite cobbles and boulders are
typically rounded to well rounded; some quartzite clasts in nearly every exposure exhibit
partial red cortexes, which may indicate derivation from El Rito Formation outcrops.
The surface of this gravel deposit projects above the Guaje-bearing terrace deposit
southwest of Medanales and thus is older. The surface of this deposit may correlate to
the higher, extensive Pliocene surfaces preserved in the Española Basin (e.g., top of Puye
Formation, top of Servilleta Basalt capping Black Mesa, and the Rio del Oso surface of
Manley, 1976); if so, this surface is likely late Pliocene in age (2-3 Ma).
Tg2
Coarse gravel preserved north of El Rito (lower Pliocene) – Sandy gravel located at
top of exposure west of the painted “E” north of El Rito. This deposit consists of very
poorly sorted, sandy gravel of mixed quartzite and felsic volcanic clasts (about 2/3
quartzite and foliate quartzite to 1/3 felsic volcanic clast). Gravel is clast-supported,
subrounded to rounded, and contains pebbles with 30-40% cobbles and 10% boulders.
Beds are vague, thin to thick, and lenticular to broadly lenticular. Weakly consolidated.
6-12 m-thick.
Tg1
Coarse gravel above eastern basalt-capped mesa on Sierra Negra (uppermost
Miocene) – Subrounded pebbles and cobbles of felsic volcanic clasts and quartzite that
appear to lie above the basalt that caps the mesa east of Sierra Negra. It has been
reported that similar gravel underlies the basalt, but we have not confirmed this. 1-2 mthick.
BASIN-FILL DEPOSITS: SANTA FE GROUP AND LOS PINOS
FORMATION
We include the Abiquiu and Tesuque Formations within the Santa Fe Group. The Abiquiu
Formation was proposed by Smith (1938) for whitish, stream-laid tuff and volcanic
Page 10 of 31
conglomerate deposits, with non-volcanic clasts dominating in its lower part (in what we refer to
as the Ritito Conglomerate). The Tesuque Formation was proposed by Spiegel and Baldwin
(1963) for Miocene basin-fill sediment, primarily pinkish-tan silty arkosic sandstone, deposited
in the Rio Grande rift near Santa Fe. Galusha and Blick (1971) later subdivided the Tesuque
Formation into several members, the pertinent ones for this quadrangle being the Chama-El Rito,
and Ojo Caliente Sandstone Members. We identify an important lithostratigraphic unit on this
quadrangle as the Plaza lithosome, which is a dacitic-andesitic conglomerate derived from an
eroded and buried volcanic center in the San Luis Basin. The term Plaza lithosome is modified
from the Plaza petrosome of Ingersoll et al. (1990), according to criteria by Wheeler and Mallory
(1956) and Bates and Jackson (1984), because we consider this unit as a recognizable
lithostratigraphic body that intertongues within other units of the Santa Fe Group of differing
lithic composition (i.e., the Chama-El Rito and the Ojo Caliente Sandstone Members of the
Tesuque Formation along with the older Abiquiu and Los Pinos Formations). The Plaza
lithosome is probably worthy of a member-rank designation. In general, the Tesuque Formation
units exposed on this quadrangle probably range in age from 20-11 Ma. Below, we discuss the
age control for each unit. Geochronologic investigations of tephras and basalts intercalated in
the Santa Fe Group are currently underway, and when complete will likely refine the age
interpretations presented here. Figure 4 shows the stratigraphic relations of the Abiquiu
Formation and Tesuque Formation lithostratigraphic units. Unless otherwise noted below,
contacts between Santa Fe Group units are conformable
In general, the Abiquiu and Tesuque Formations on this quadrangle were deposited by streamflow processes on an alluvial slope or alluvial fan. The depositional surface generally sloped to
the southwest, as indicated by the abundant paleoflow data collected on this quadrangle (Figure
5), but later in Tesuque Formation deposition (i.e., deposition of unit Ttpu and overlying Ttc,
age range of 15-13.4 Ma) the general slope had changed to the southeast. The Abiquiu
Formation was deposited in wide channels that avulsed or shifted laterally with time. It was
derived from erosion and transport of detritus from the 24-28 Ma Latir volcanic field to the
northeast, located in the south-central San Luis Basin (Smith et al., 2002; Zimmerman, 2007).
The sediment from this volcanic field is primarily felsic. A younger dacitic-andesitic volcano
developed in the south-central San Luis Basin at 22-23 Ma, based on dating of gravel by Ekas et
al. (1984) and discussed in Ingersoll et al. (1990). Erosion and southwest transport of detritus
from this volcanic field approximately coincides with the initiation of orange fine sand
deposition that characterizes the Chama-El Rito Member and concomitant Plaza lithosome of the
Tesuque Formation. Since poor exposures in most of the quadrangle do not offer adequate
assessment of the presence and amount of the fine orange sand, we use the lowest thick interval
of the distinctive dacite-andesite gravel derived from this 22-23 Ma volcano to map the base of
the Tesuque Formation. The aforementioned change of paleoflow to the southeast beginning ca.
15 Ma (in units Ttpu and overlying Ttc in the eastern quadrangle) is interpreted to be a
consequence of increased subsidence rates along the Ojo Caliente fault east of this quadrangle
(refer to the Ojo Caliente geologic map by Koning et al., 2005b). The increased hetereolithic
character of these units is interpreted to be a result of increased tectonic tilting and related uplift
of the southern Tusas Mountains, as discussed below. The stratigraphically highest and youngest
Tesuque Formation unit is the Ojo Caliente Sandstone. The thick cross-stratification, which
generally dips to the northeast, and well-sorted, quartz-rich sand (fine- to medium-grained) is
consistent with deposition in an eolian dune field, as has been interpreted in Galusha and Blick
(1971) and May (1980). It is important to note that the Ojo Caliente Sandstone interfingers with
Page 11 of 31
the upper Chama-El Rito Member in the western part of the quadrangle, as illustrated in Figure
4.
The Los Pinos Formation was redefined by Manley (1981) as consisting of an Esquibel Member
(derived from the San Juan Mountains) and a felsic gravel-bearing Cordito Member (derived
from the Latir volcanic field). The Cordito Member is present on this quadrangle, and bears a
distinctive gravel assemblage consisting primarily of gray, banded rhyolite and reddish brown
crystalline rhyolite, with subordinate tuff clasts (welded and non-welded). Manley (1981) does
not include the Los Pinos Formation in the Santa Fe Group.
Tto
Ojo Caliente Sandstone Member of the Tesuque Formation (middle Miocene) –
Very pale brown (10YR 7/3-8/2), cross-stratified sandstone. Sand is typically upper-fine
to lower-medium grained, well-sorted, subrounded to subangular, and consists of quartz,
minor possible plagioclase, 15% possible potassium feldspar, 3-7% felsic to intermediate
volcanic grains, and 3-7% mafic grains. Redder (oranger) hues are present near known or
inferred faults, where cementation is also conspicuous. The largest outcrop area is north
of Alamosa Canyon and west of Alamosa No. 2 Tank, where roughly 30 m of nearly flatlying (probably gently NE dipping) sandstone is moderately cemented and forms a
dissected mesa. Cross-bedding is clearly apparent in this outcrop belt and cross-bed sets
are typically 3-5 m thick; wind directions range from southwest to slightly northwest
(cross bed dips toward 35o-105o). Orange, lithified outcrops of Tto also form
conspicuous buttes, labeled with elevations 6926 and 7022 just east of the Tg3
escarpment; these cemented outcrops are in the proximal hangingwall of a mapped fault.
Tto was also mapped for discontinuous exposures of unlithified, well-sorted, fine- to
medium-grained sand mantled in quartzite-gravel colluvium both north and south of the
mouth of canyon through which State Road 137 passes. A thin interval of this unit is
preserved on the hanging-wall of an east-down fault in the southeast corner of the
quadrangle. Unit gradationally overlies the Chama-El Rito Member, and interfingers
with the upper Chama-El Rito Member in Alamosa Canyon (as illustrated in Figure 4).
Weakly to moderately consolidated where not cemented. Preserved thicknesses are
greatest (50 m) in upper Alamosa Canyon.
Direct age control for this unit is not available on this quadrangle. To the southeast of the
quadrangle, the Pojoaque white ash zone is interpreted to extend into the basal part of this
unit (Koning, 2004). The Pojoaque white ash zone has an interpreted age of 14.0-13.2
Ma (Koning et al., 2005c, and Koning, 2002, based on data from Izett and Obradovich,
2001, Barghoorn, 1981, Tedford and Barghoorn, 1993, and the revised geomagnetic
polarity time scale of Cande and Kent, 1995). We follow Koning et al. (2005a) in using
13.4 Ma as the maximum age of this unit. An interpreted 11.3 Ma Trapper Creek ash is
present in the upper Ojo Caliente Sandstone Member under southern Black Mesa (Nelia
Dunbar, written commun., 2005; Koning, 2004). Thus, the age range of the Ojo Caliente
Sandstone is interpreted to be approximately 13.4-11.0 Ma.
Page 12 of 31
Ttcf
Chama-El Rito Member, Tesuque Formation, fine-grained deposits (middle
Miocene) – Pink to very pale brown (7.5-10YR 7/4) to reddish yellow (7.5YR 6/6), very
fine- to fine-grained sand (locally lower-medium sand) and silty sand. Pink to orange
colors seem to be most common. Strata are typically in medium to thick, tabular beds
that are internally massive or planar-laminated. Sand locally has minor medium- to very
coarse-grained volcanic grains and very fine pebbles, either scattered or in very thin
lenses. Sand is subangular to subrounded, moderately to well-sorted, and consists of
quartz, minor plagioclase(?), 15-25% potassium feldspar and orange-stained quartz, 1-5%
volcanic grains, and 3-7% mafic grains. 1-5% interbeds of brown to light brown (7.5YR
5-6/4) to reddish brown (5YR 4/3-4) clay and clayey-silty very fine- to fine-grained sand;
1-3% interbeds of pink to very pale brown (7.5-10YR 7/4) siltstone. Minor interbeds of
planar-bedded or trough-cross bedded, medium to coarse sandstone with dispersed
granules of volcanic rock types. Unit may also include minor pebbly channel-fills similar
to units Ttpm or Ttpu. This unit is most conspicuous along the margin of the Tg3 mesa
north of Sierra Negra and east of Madera Cañon, where it is more than 30 m thick. It also
underlies Tto on the 6926 butte and is locally well exposed in sections up to 20 m thick in
the unnamed arroyo valley east of 6926 butte. Unit overlies Ttc in the western part of the
quadrangle, but these strata are probably equivalent in age to the Ttc in the eastern part of
the quadrangle (where the latter is mapped overlying Ttpu). Unit is mapped as tongues
within unit Ttpm in the eastern part of the quadrangle. Beds of this unit are common
(15-50% of sediment volume) in unit Ttpu, but were not differentiated there. Moderately
to well consolidated and generally non-cemented (localized strong cementation). Unit is
equivalent in age to Ttpm and Ttpu, and it minimum age may be slightly younger than
the latter. These units are interpreted to be 16-13.4 Ma, as discussed below. Maximum
preserved thickness of 55 m, but individual tongues mapped within unit Ttpm may be as
little as 6 m-thick.
Ttc
Chama-El Rito Member, Tesuque Formation (lower to middle Miocene) – Pink to
orange, fine-grained sandstone, silty sandstone, and siltstone interbedded with gray
pebble gravel. Unit assigned for thick intervals (greater than 30 m) where very fine- to
medium-grained sand is subequal to or exceeds volcaniclastic, coarse channel-fills. Unit
contains more volcaniclastic channel-fills than Ttcf. Fine sand is similar to that
described in unit Ttcf above. Coarse channel-fills tend to be 1-2 m thick and consist of
tightly packed gravel with subangular to subrounded clasts generally 1-4 cm and rarely
larger than 10 cm across. Visual estimates of composition indicate about 5-10% quartzite
and the remainder volcanic pebbles. The volcanic pebbles are overwhelmingly light-gray
to pink, and in some cases white, hornblende-biotite-plagioclases rocks, suggesting
andesitic to dacitic composition; more felsic clasts are very scarce or absent, but exceed
15% in the eastern part of the quadrangle above the Ttpu unit. Although the orange-andgray striped character of Chama-El Rito outcrops is typical, both the fine and coarse
layers are distinctly whiter in the vicinity of faults north of Sierra Negra. This unit
gradationally overlies Ttpu in the eastern quadrangle. Thickness on the east margin of
the quad decreases northwards from approximately 120 m to 50 m. Unit is at least 250
m-thick in the southwestern part of the quadrangle, and possibly up to 500(?) m-thick.
Page 13 of 31
Direct age control for this unit is not available on this quadrangle. Ttc mapped near the
eastern quadrangle boundary overlies Ttpu, which may contain part of the Pojoaque
white ash zone. If that is correct, then Ttc there would probably be 13.7-13.4 Ma,
considering the age assignment of the Ojo Caliente Sandstone. The Pojoaque white ash
zone has an interpreted age of 14.0-13.2 Ma (Koning et al., 2005c, and Koning, 2002,
based on data from Izett and Obradovich, 2001, Barghoorn, 1981, Tedford and
Barghoorn, 1993, and the revised geomagnetic polarity time scale of Cande and Kent,
1995). However, Ttc extends into much lower stratigraphic levels near the westsern
quadrangle boundary. Here, it extends below unit Tpt (inferred age 15-16 Ma, as
discussed below). Tedford and Barghorn (1993) suggest a maximum age of 16-18 Ma
based on fossil data. Based on our inferred age of lithosome P, we suspect that this unit
may be as old as 20 Ma.
Ttpu Upper unit of Plaza lithosome, Tesuque Formation (middle Miocene) – Coarse
channel-fill complexes (up to several meters thick) of pebbly sand to sandy gravel,
possessing greater than 10% felsite clasts, intercalated with fine-grained, pink to orange
sand. Gravelly sediment is clast- to sand-supported and in laminated to very thin to
medium beds. These beds are broadly lenticular to planar (most common), but locally
lenticular and planar-cross-stratified (up to 2 m-thick foresets). Gravel includes pebbles
with 5-7% cobbles and 1% boulders, and appears to have a coarser size range than the
underlying Ttpm unit. Clasts consist predominately of dacite-andesite clasts with 1050% rhyolite and felsic tuff clasts (welded and non-welded, includes Amalia Tuff), 1-5%
intermediate intrusive clasts (e.g., graniodiorite, quartz diorite, and tonalite -- weathered
intermediate clasts have a slightly greenish color), 1-3% orange granite, 1-5% quartzite,
and 1-3% basalt. Channel-fill sand may have orangish gray (pinkish gray to light reddish
brown light brown to reddish yellow, 5-7.5YR 6/2-6) to gray hues (light brownish gray to
light gray to gray, 10YR 6/1-3, 6-7/2). Channel-fill sand is poorly sorted and very fineto very coarse-grained. Upper-medium- to very coarse-grained sand consists of
subrounded volcanic grains with minor subrounded to subangular quartz and plagioclase
grains. Very fine to lower-medium sand is dominated by sand similar to unit Ttcf;
locally, sand is tuffaceous (i.e., orangish clay presumably a weathering product of ash).
Generally non-cemented (about 10% strong to moderate cementation) and weakly to
moderately consolidated. The lesser degree of consolidation and cementation of this unit
compared to the underlying Ttpm unit commonly results in a topographic slope decrease
near its basal contact. In the northeastern quadrangle, the base of this unit was drawn
above Ttpm where the percentage of felsite clasts exceeded 10%. However, in the
southeastern quadrangle, the upper 20-30 m of the middle unit of the Plaza lithosome
(Ttpm) contains appreciable felsite clasts (at or above 10%). Because this gradation was
difficult to identify precisely, we mapped the base of the upper unit above this gradation;
specifically, at the base of a thick, orangish, fine sand interval above which fine sands
occupied greater than 15% of the strata (over about a 30 m stratigraphic interval). Unit
interfingers with the Chama-El Rito Member a short distance south of the quadrangle’s
south boundary, and gradationally underlies this member on this quadrangle. Unit
gradationally(?) overlies Ttpm in the eastern part of the quadrangle. Near the
northeastern corner of the quadrangle, this unit may interfinger northeastward with a unit
Page 14 of 31
similar to Ttpml, but poor exposure there makes this inference uncertain. 120-150 mthick.
White, fine ash beds were found in this unit in the southeastern corner of the quadrangle
(UTM coordinates of: 398215 E, 4012819 N and 397800 E, 40039920 N, NAD 27).
These are about 10 to 30 cm-thick and altered. Also, rip-ups clasts consisting of
consolidated ash have been located in channel-fills in this unit (UTM coordinates
(NAD27): 397869 E, 4012359 N; 397520 E, 4012465 N; 397169 E, 4012215 N). These
fine tephra probably correlate to the Pojoaque ash zone, considering this unit’s
stratigraphic position below the Ojo Caliente Sandstone (Tto) and 200-260 m above the
Tpt marker unit east of El Rito Creek. Unit Tpt is interpreted to be 15-16 Ma, and thus
approximately equivalent in age to the Skull Ridge ashes east of Espanola, also referred
to as the early Barstovian tuffaceous interval (Koning et al., 2005c and 2007; Izett and
Obradovich, 2001; Barghoorn, 1981). The Pojoaque white ash zone has an interpreted
age of 14.0-13.2 Ma (Koning et al., 2005c, and Koning, 2002, based on data from Izett
and Obradovich, 2001, Barghoorn, 1981, Tedford and Barghoorn, 1993, and the revised
geomagnetic polarity time scale of Cande and Kent, 1995). Considering these
constraints, the age of this unit is probably within 15-13.5 Ma.
Ttpw Western tongue of Plaza lithosome, Tesuque Formation (middle Miocene) – Tongue
of volcaniclastic conglomerate mapped between lower Arroyo del Perro and Arroyo del
Perro del Oeste. Similar to unit Ttpm in color, bedding, and texture, but with an
estimated 10-20% welded tuff, tuff, and minor rhyolite clasts. Other minor clasts
include: trace to 1% intermediate intrusives (e.g., granodiorite, tonalite, quartz diorite),
trace to 1% orange granite, trace basalt, and trace quartzite. Remainder of clasts consists
of light to dark gray to purplish gray, commonly porphyritic dacite to andesite. Unit
probably correlates with Ttpm to the east. The slight difference in clast composition
between this unit and Ttpm is attributed to a more western stream eroding slightly
different older sediment in what is now the Tusas Mountains. Age range of unit is
probably similar to that of Ttpm (16-14 Ma). About 30 m-thick.
Ttpm Middle unit of Plaza lithosome, Tesuque Formation (middle Miocene) – Gray to
brownish gray to pale brown(10YR 6/1-3), slightly tuffaceous (estimate 1-5% tuff),
sandy pebbles and pebbly sand in very thin to medium, lenticular to broadly lenticular to
planar beds. Local tangential- to planar-cross-stratification in very thin to thin beds;
about 10% U-shaped, discrete channel-fills 10-50 cm-thick. Cross-stratification seems
more common in this unit than in the underlying Ttpl unit. Gravel are matrix- to clastsupported and consist of very fine to very coarse pebbles and minor cobbles (~5%); the
majority of pebbles are very fine to medium. Clasts consist of purplish gray, porphyritic
dacite to andesite, in which many of the coarser pebbles have greater than 10%
plagioclase phenocrysts up to 8 mm-long in addition to 3-10 % hornblende and biotite up
to 3 mm. Other clasts include light gray, white, and pinkish white dacites that often are
weathered and have 5-15% mafic phenocrysts (typically hornblende and minor biotite)
Page 15 of 31
and less than 15% plagioclase phenocrysts, 1-12% quartzite, and 1-15% rhyolite and
welded tuffs (increasing up-section in the middle to upper part of the unit). Channel-fill
sand is gray to light gray to pinkish gray (7.5-10YR 6-7/1-7/2) and very fine to very
coarse-grained (mostly medium- to very coarse-grained). Medium to very coarse sand is
subrounded to subangular and a litharenite. Finer sand is similar to that in unit Ttcf but
with greater plagioclase and as much as 20% orangish clay (probably altered tuff). Ttpm
grades upward into Ttpu and laterally (southward) into unit Ttc; this southward
transition appears to occur just south of the southern border of the quadrangle. Most of
unit has less than 15% fine sand beds similar to unit Ttcf (over a given >30 m
stratigraphic interval, excluding mapped tongues of unit Ttcf); however, the lower 20-30
m has as much as 40-50% of these fine sandstone interbeds . Unit corresponds to Los
Pinos tongue of May (1980 and 1984). Base of unit placed above the highest beds of
Tpt, Ttcfl and Ttpl in the southeastern quadrangle. In the northeastern part of the
quadrangle, where unit Tpt is absent, the lower and middle parts of lithosome P are
similar and lumped in a combined unit (Ttpml) that is lithologically similar to what is
described here. Well consolidated and weakly to moderately cemented by calcium
carbonate and tuff, locally producing cliffs and ledges in the landscape. This unit
overlies the Tpt marker unit east of El Rito Creek, which is inferred to be 15-16 Ma, and
so we interpret the age range of Ttpm as 16-14 Ma. Approximately 200-260 m-thick.
Ttpml Undivided lower to middle Plaza lithosome, Tesuque Formation (lower to middle
Miocene) – Lithologically similar to the middle lithosome P unit (Ttpm), but extends
down-section past the projected phreatomagmatic interval (Tpt) and so includes strata
probably time-equivalent to Ttpl. Age range of 20-14 Ma based on the interpreted ages
of the individual Ttpm and Ttpl units and the age ranges of clasts collected from the
Plaza lithosome (Ekas et al., 1984; Ingersoll et al., 1990). 300-400 m-thick.
Ttcfl – Chama-El Rito Member, Tesuque Formation, lower fine-grained deposits (lower to
middle Miocene) – Very pale brown (10YR 7/3) to light gray (10YR 7/2) to pinkish gray
and pinkish white (7.5YR 7-8/2) to pink (7.5YR 7-8/3), fine-grained sand and silty fine
sand deposits below unit Tpt east of El Rito Creek. Mapped above the Ttpl unit or in the
upper part of that unit. These deposits are lighter colored, better consolidated and
cemented, and commonly exhibit more distinctive bedding than fine-grained intervals upsection. The lighter color and cementation may be partly due to circulation of warm
groundwater following emplacement of dikes and phreatomagmatic volcanism in the
middle Miocene. Strata generally consist of thin to thick, tabular to broadly lenticular
beds of very fine- to fine-grained sandstone and silty sandstone (locally lenticular and
trough-cross-stratified). Beds are internally massive to planar-laminated to trough crossslaminated (also very thin-bedded). Sand is very fine- to fine-grained, subangular to
subrounded, moderately to well-sorted, and consists of quartz, plagioclase, 15-25%
potassium feldspar and orange-stained quartz, 5-18% biotite and other mafic grains, and
1-10% volcanic grains. Locally scattered in the sand are medium to coarse, dacitic(?)
sand grains. Sand is locally very slightly tuffaceous and weakly to well cemented.
Page 16 of 31
Minor (3-15%) very thin beds or planar-laminations of light brown to reddish yellow
(7.5YR 6/3-6) to light reddish brown (5YR 6/4-6) clay and clayey very fine- to finegrained sandstone. Unit includes subordinate volcaniclastic, coarse channel-fills similar
to those described in unit Ttpl. Age of unit immediately predates that of the overlying
Tpt, so it is probably around 16 Ma. 30-50 m-thick.
.
Ttpl
Tlpc
Lower unit of Plaza lithosome (lower to middle Miocene) – Light-colored, weakly
altered, and weakly to strongly cemented channel-fill complexes interbedded with
subordinate fine sandstone intervals similar to unit Ttcfl. Occurs below unit Tpt east of
El Rito Creek. Channel-complexes include pebbly sandstone and sandy pebble
conglomerate. Beds are planar to lenticular to cross-stratified (up to 50 cm-thick
foresets), and laminated to very thinly to medium-bedded (minor thick-bedded). Crossstratification seems to be less common than higher in the Plaza lithosome. Gravel are
clast-supported, subrounded (mostly) to subangular, poorly to moderately sorted, and
consist of very fine to very coarse pebbles with 3-5% cobbles. Clasts are dominated by
altered dacite-andesite, with 1-5% quartzite, 1-10% rhyolite and welded tuff (Amalia
Tuff is absent to very sparse), trace granite, trace gneiss, and trace to 1% basalt; locally,
non-welded tuff is as much as 15%. Dacite clasts are lighter than those higher in
lithosome P and possibly have slightly less coarse plagioclase phenocrysts, but otherwise
mineral assemblages are similar and the apparent difference may be partly due to
alteration from hydrothermal(?) waters responsible for the varying degrees of silicacementation (minor calcite) of this unit. Channel-fill sand is typically light gray (10YR
7-8/1-2) to very pale brown (10YR 8/2), fine- to very coarse-grained, subangular to
subrounded, poorly to moderately sorted, and a volcanic litharenite. Fine sand contains
subequal volcanic grains compared to quartz and plagioclase, with 10-15% potassium
feldspar and orange-stained quartz; medium to very coarse sand generally consists of
volcanic grains. Sand is locally tuffaceous. Unit underlies the Tpt marker unit east of El
Rito Creek, interpreted to be 16-15 Ma, and extends to the base of the Plaza lithosome.
This lithosome must post-date the age of the dacite-andesite gravel sampled from it (~2223 Ma; Ekas et al., 1984) and must post-date our interpreted ages of the Los Pinos and
Abiquiu Formations, so we assign an age of this unit of 20-15 Ma. More than 120 mthick.
Cordito Member of the Los Pinos Formation (upper Oligocene to lower Miocene) –
Gravelly sand, sandy gravel, and sand beds. Gravelly sediment is in very thin to medium,
broadly lenticular to lenticular beds that may be internally planar-laminated. Gravel is
composed of rhyolite (including minor possible rhyodacite) with subordinate welded and
non-welded tuff (including Amalia Tuff), trace to 5% quartzite, and minor dacite
(generally porphyritic). Rhyolite clasts are commonly banded and grayish, or crystalline
with quartz phenocrysts. The rhyolite and tuff clasts serve to distinguish this unit from
the overlying Plaza lithosome of the Tesuque Formation. Gravel consists of pebbles and
cobbles with very minor boulders. Sand is commonly in medium to thick, tabular to
broadly lenticular beds. Common sand colors include pinkish gray, pink, light brownish
Page 17 of 31
gray, and light gray (7.5YR 7/2-3 and 10YR 6/2 and 10YR 7/1-2); sand is very fine- to
very coarse-grained (mostly fine- to very coarse-grained), subrounded to subangular,
poorly sorted, and consists of plagioclase and quartz with minor felsic volcanic grains,
mafic grains, and very minor (1-3%) possible potassium feldspar. Estimate 0-5% tuff.
On the quadrangle, this unit gradationally overlies the more tuffaceous, white-colored
Abiquiu Formation and gradationally underlies the grayer, less felsic Plaza lithosome
(Tesuque Formation). Upper contact placed where felsite clasts (i.e., rhyolite and felsic
tuff clasts) are less than 10% of the gravel assemblage. In the quadrangle to the north,
this unit is mapped lower in the section and interfingers southwestward with the Abiquiu
Formation. Unit is well consolidated and non-cemented. Approximately 30-60 m-thick.
The upper Cordito Member on the Valley Grande Peak quadrangle to the north forms a
mappable unit, commonly capping ridges, that overlies the Abiquiu Formation (the lower
to middle Cordito Member interfingers with the Abiquiu Formation; Kempter et al.,
2008). We interpret that the same upper part of the member extends into this quadrangle.
In upper Arroyo Seco on the Valle Grande Peak quadrangle, the upper Cordito Member
overlies a basalt flow in the upper Abiquiu Formation that has been K/Ar dated at ~20.7
Ma (Manley and Mehnert, 1981). This basalt is being re-dated using 40Ar/39Ar, but for
the time being we use this date to assign a preliminary 20-21 Ma age range for the
Cordito Member that extends onto this quadrangle.
Ta
Abiquiu Formation (lower Miocene to upper Oligocene) – Tuffaceous, pebbly
sandstone, sandstone, and clayey-silty sandstone with minor conglomerate; colors range
from white and beige to red-brown to orange near faults north of Sierra Negra. Volcanic
clasts include a variety of intermediate to felsic rocks types, with rhyolitic clasts
prominent in all outcrops and clasts of Amalia Tuff seen at numerous localities. Volcanic
boulders, 25-60 cm across, are prominent in many outcrops. Quartzite pebbles and
cobbles are commonly present as well; quartzite typically composes less than 10% of the
gravel size class but some beds contain >90% quartzite.
This formation comprises the bulk of the hills north of El Rito. Here, the Abiquiu
Formation consists of sand, pebbly sand, and silty-clayey sand intercalated with minor
clast-supported, sandy gravel beds. Strata are in very thin to thick, tabular to broadly
lenticular beds that are commonly planar-laminated and locally cross-laminated. Gravel
here includes pebbles and local cobbles plus minor boulders. Clasts are subrounded to
subangular, moderately to poorly sorted, and composed of welded tuff and tuff, rhyolite,
and 1-20% quartzite. At some localities, gravel includes 1% of a yellowish-orangish,
weathered tuff, 1-10% dacite and rhyodacite, 1-2% muscovite- biotite-granite, 1-10% of
an orange, Proterozoic metarhyolite, and trace to 1% basalt. Welded tuff includes Amalia
Tuff, which locally comprises as much as 15-20% of the gravel fraction. Minor beds of
gravel consisting mostly of quartzite with 1-3% granite, 1-10% foliated quartzite, and 35% felsite clasts. Sand is white (2.5Y-10Y-7.5R 8/1) to light gray (10YR 7/1-2),
commonly but not pervasively tuffaceous (1- 20% tuff), fine- to very coarse-grained
(mostly medium- to coarse-grained), moderately to poorly sorted, and a mixture of
subangular (mostly) to angular plagioclase and quartz, with minor (generally <10%)
Page 18 of 31
subrounded to subangular felsic volcanic grains, 0-7% possible potassium feldspar, and
1-10% mafic grains (euhedral, fresh-appearing biotite common). Silty-clayey sand is
pink to light brown (7.5YR 6-7/3, and generally has an estimated 1-5% fines.
Moderately to well consolidated and weakly to moderately cemented by tuff. In the
quadrangle to the north, and partly on this quadrangle, this unit interfingers northeastward
into the Cordito Member of the Los Pinos Formation. Unit overlies the Ritito
Conglomerate on this quadrangle, and grades upward into the Cordito Member of the Los
Pinos Formation. Greater than 250 m-thick, except on the immediate footwall of the
Potrero fault.
We interpret an age range of 26-20 Ma for the Abiquiu Formation (excluding Vazanna
and Ingersoll’s (1981) lower member of the Abiquiu Formation, which we call the Ritito
Conglomerate, and the Pedernal Chert). 40Ar/39Ar dating of volcanic gravel near Abiquiu
indicates that the lowest strata of this formation there are likely 25-26 Ma (Smith et al.,
2002). We prefer a minimum age of ~20 Ma for this formation for four reasons. First, a
basalt flow interbedded in the upper Abiquiu Formation in upper Arroyo Seco, located on
the Valle Grande Peak quadrangle to the north, returned a K/Ar age of ~20.7 m.y.
(Manley and Mehnert, 1981). This basalt is currently being re-dated by 40Ar/39Ar
methods. Second, Koning et al. (2005b) re-interpreted the olivine nephelinite at Cerro
Negro (located Ojo Caliente quadrangle to the east) as a small intrusion, rather than a
flow as interpreted by Baldridge et al. (1980). This was K/Ar-dated at 18.9 +/- 0.7 m.y.
(Baldridge et al., 1980) and intrudes the upper Abiquiu Formation there. Thus, the upper
Abiquiu Formation there predates 19 Ma. Third, fossils collected in the upper Abiquiu
Formation are late Arikareean and thus 20-23 Ma in age (Tedford and Barghoorn, 1993).
Fourth, extrapolation of the age of the Abiquiu Formation using the deposition rate
shown by Smith et al. (2002, fig. 12) suggests this formation is older than ~20 Ma. The
only evidence rebuffing a 20 Ma minimum age is a reported 17.3 +/- 0.8 m.y. K/Ar age
on a dacitic clast approximately 10 m below the top of the Abiquiu Formation, at a
location 7 km northeast of Abiquiu (Manley and Mehnert, 1981; May, 1984). Although
we have not visited that locality, consideration of the gravel composition of the upper
Abiquiu Formation on this quadrangle (predominately felsic clasts) suggests the
possibility that this clast may have come from strata correlative to what we call lithosome
P on this quadrangle.
Talpc Undivided Abiquiu-Los Pinos Formations (upper Oligocene to lower Miocene) –
Probable interfingering Abiquiu and Los Pinos Formations (Corditto Member), where
exposure was too poor to differentiate. See descriptions of the two units above.
Approximately 30 m-thick where mapped on this quadrangle (near the northern
boundary).
Tr
Ritito Conglomerate (Oligocene?) – Quartzite-rich pebble to cobble gravel with lenses
and layers of white, quartzofeldspathic sand as much as 8 m thick. This unit was mapped
in Angel and Stone Canyons, in addition to the footwall of the Potrero fault north of El
Page 19 of 31
Rito (where boulders are present in the gravel fraction). Gravel always forms slopes, so
bedding character is not obvious near the western quadrangle boundary. North of El
Rito, a prominent arroyo immediately west of the painted “E” offers exposures of
medium to thick, tabular to irregular beds of sandy gravel and pebbly sand. Here,
pebbles, cobbles, and boulders are matrix-supported, subrounded, poorly sorted, and
consist of quartzite (boulders and cobbles are exclusively quartzite) and felsic volcanic
rocks (i.e., rhyolite and tuff), along with trace basalt. Boulders decrease up-section.
Matrix consists of white (10YR 8/1), fine- to very coarse-grained sand that is tuffaceous,
poorly sorted, and consists of quartz, 20-40% felsic volcanic grains, and 1-2% fresh
biotite crystals). In this exposure, the contact with the overlying Abiquiu Formation
appears conformable.
.
Near the western quadrangle boundary, clast sizes are dominantly 3-25 cm with scattered
clasts as large as 50 cm. Rounded to well rounded clasts are overall finer than in Tg3 and
better rounded than in El Rito Formation (Ter). The white sand contains several percent
magnetite and biotite and forms low, ~1-1.5 m high outcrops in a few arroyo headcuts
within the recently burned part of Angel Canyon. An 8-m thick sand interval crops out in
Stone Canyon (388988 E, 4025770 N; zone 13, NAD 27) and is white to light gray with
thin, orange silty-sand intervals. At this outcrop, scattered small pebbles are
overwhelmingly quartzite, although a few intermediate-volcanic pebbles up to about 1 cm
across were noted. In both Angel and Stone Canyons, the Ritito Conglomerate contains
10-15% orange granite and metarhyolite, 1-2% white vein quartz, and the remainder is
quartzite. Unit is correlative to the Ritito Conglomerate of Barker (1958) and the lower
member of the Abiquiu Formation (Vazzana and Ingersoll, 1981; Moore, 2000; Smith,
1995). Unit unconformably overlies the El Rito Formation. There are no direct age
constraints on this unit, but it is very likely Oligocene-age since it predates the Abiquiu
Formation (26-20 Ma) and post-dates the Ritito Conglomerate (Eocene?). Weakly to
moderately consolidated and non-cemented. 80-90 m-thick.
Ter
El Rito Formation (Eocene?) – Red, lithified conglomeratic sandstone, sandstone, and
mudstone. Well exposed in a horst block in Angel Canyon and poorly exposed along the
base of Stone Canyon. Clasts are >90% quartzite and the remainder are vein quartz,
granite, and metarhyolite. Clasts are subangular to rounded and range in size up to 1 m
across. The lithified character, red color, and more angular clasts distinguish the El Rito
Formation from the overlying Ritito Conglomerate. The age of these sediments is
regarded to be Eocene (Logsdon, 1981; Smith et al., 1961) and the unit is thought to have
been deposited in a syn-orogenic basin between Laramide highlands. 30 m-thick where
on quadrangle, as much as 60 m-thick in the southern Valle Grande Peak quadrangle to
the north (Kirt Kempter, written commun., 2008).
VOLCANIC ROCKS
Page 20 of 31
Tb
Basalt flows (Miocene-Pliocene) – Basalt flows occur at two localities. Flows that cap
the summit and eastern mesa of Sierra Negra are sparsely pophyritic basalt with laterally
variable textures. Phenocrysts consist of plagioclase and 1-4% olivine. Basalt on the
summit is only 3-6 m-thick. The eastern mesa is underlain by as many as 3 basalt flows
having a cumlative thickness of 20-25 m immediately west of the Madera Cañon fault
that crosses the central part of the mesa, and a cumulative thickness of 30-35 m
immediately east of this fault (including rubble zones between the flows). The second
locality is the low hill southeast of Sage Tank composed of fine-grained basalt containing
~10% olivine phenocrysts less than 1 mm across. The hill is surrounded by younger Tg3
gravel and almost certainly is within a crater defined by the distribution of
phreatomagmatic tuff outcrops to the southwest and east (Tpt). Vesicular zones suggest
the presence of lava flows such that this hill may represent a small shield volcano within
the hydromagmatic tuff ring crater; alternatively this hill could be an eroded plug within
the crater. The basalt at the second locality is probably middle Miocene in age (40Ar/39Ar
age is pending). The basalt flows on Sierra Negra have returned a K/Ar age of ~5 Ma
(Baldridge et al., 1980) and a 40Ar/39Ar age of 5.56±0.12 Ma (Maldonaldo and Miggins,
2007).
Tbi Basalt dikes and plugs (Miocene to Pliocene) – Largest dike consists of en echelon
segments that form ridges along the western quad boundary just east of Madera Cañon.
This basalt is a fine-grained, olivine-phyric rock that is indistinguishable from the Tb hill
southeast of Sage Tank. Also shown is a small part of a 1-m wide, highly altered, finegrained basaltic dike that strikes toward about 30o and is located at 39039 E, 4015212 N
(zone 13, NAD 27). Another dike was encountered but deemed too small to map; this is
a 30-cm wide, NE-striking, olivine and plagioclase phyric dike encountered at 389032 E,
4014805 N (zone 13, NAD 27). This map unit also includes a plug on the northern flank
of Sierra Negra; this rock contains 2-5% olivine and ~1-2% plagioclase phenocrysts 0.5-2
mm across. Lastly, a 0.5-3.0 m-wide dike was mapped 0.5 km east-southeast of CCC
spring; this dike has 2-3% black olivine and 25% black-green pyroxene(?) crystals in a
black, aphanetic groundmass (both 0.1-1.0 mm). An 40Ar/39Ar date on this dike is
pending.
Tpt
Phreatomagmatic tuff and lapilli tuff – Pale green to tan, and locally orange, planarbedded and cross-bedded phreatomagmatic tuff. Thickest deposit is about 30 m-thick
below the escarpment SW of Tuck Tank and exhibits easterly to southerly transport,
based on cross bedding, which is consistent with derivation from a vent centered beneath
the basalt hill southeast of Sage Tank. These deposits also likely correlate to a small
outcrop area in an unnamed canyon southwest of the basalt hill, where a steep contact
with Ttc implies the margin of a crater filled with inward-dipping tuff. Outcrops in the
headwaters of Arroyo del Perro may correspond to the same event, and similar tuffs are
found near the top of Sierra Negra. East of El Rito Creek lies a 3-30 m-thick interval of
phreatomagmatic deposits between units Ttpm and Ttcfl. These deposits are a slightly
Page 21 of 31
greenish, white to very pale brown (10YR 8/1-2), and consist of sandstone and gravelly
sandstone that are cross-stratified in various angles. Sand is very fine- to very coarsegrained, subrounded to subangular, moderately sorted, and consist of a mix of minor,
altered, basaltic sand with more abundant sand similar to that seen in units Ttpl and
Ttcfl. Gravel consists of basalt, dacite-andesite pebbles of the Tesuque Formation, and
minor Proterozoic clasts. This unit east of El Rito Creek appears to correlate to the
basaltic eruptive complex straddling El Rito Creek 1 km south of the south boundary of
the quadrangle (Koning et al., 2004). A basalt bomb in this complex, sampled east of El
Rito Creek, was dated at 15.27 +/- 0.39 m.y. by K/Ar methods (Ekas et al., 1984). Thus,
the age of Tpt east of El Rito Creek is interpreted to be 15-16 Ma. Well consolidated and
locally cemented.
A note about basalts and phreatomagmatic units: Although a single set of map units
was used for basaltic lava, intrusions, and tuff in the field, these units are not all
correlative. Given previous K-Ar and 40Ar/39Ar dates implying a Pliocene age for the
basalt capping Sierra Negra (Baldridge et al., 1980; Maldonaldo and Miggins, 2007), we
assume that the Sierra Negra basalt is unrelated to the basalts long recognized within the
Chama-El Rito Member of the Tesuque Formation. The Tbt beds in the headwaters of
Arroyo del Perro are clearly in the Ttc. The basalt hill and related Tpt outcrops west of
Tuck Tank are not clearly overlain by Ttc and could arguably be younger than Ttc and
older than Tg3. However, we are inclined to interpret this eruptive center as being coeval
with Ttc. The basaltic tuff in the summit region of Sierra Negra is separated from the
summit basalt by about 7 m of orange sand and volcanic-pebble gravel that is almost
certainly Ttc. The basalt plug on the north side of Sierra Negra probably marks the vent
for the basalt capping the mesa east of Sierra Negra, whereas the thin flow capping the
summit does not seem to correlate to these other two basalts (see Appendix 1). It is
difficult to discern whether the various dikes are related to the Pliocene-age Sierra Negra
basalt or to older, Chama-El Rito vintage volcanism. The en-echelon dikes just east of
Madera Cañon curve toward the basalt hill that projects above the Tg3 surface southeast
of Sage Tank, and we infer that they are middle Miocene in age.
PROTEROZOIC ROCKS
Xoq
Ortega Quartzite (Paleoproterozoic) – Light gray (N/7), coarse-grained, vitreous,
laminated quartzite. Laminations are planar, wavy, to low-angle cross-stratified. ~5%
lenticular pods of vein quartz. Biotite, as well as aligned silliminite, are concentrated in
primary foliation planes, which themselves appear to follow original bedding. Locally
present are flattened, very fine to very coarse quartz pebbles that are shortened
perpendicular to the primary foliation planes. 60 m of exposed thickness on this
quadrangle, but total thickness is much greater.
Page 22 of 31
DEEP STRATIGRAPHIC UNITS DEPICTED ON CROSSSECTION BUT NOT LISTED ON MAP
lPPcu Undivided Cutler Group – Red to orange to brown siltstone, sandstone, and
conglomerate. Sand consists of commonly medium- to coarse-grained, moderately to
poorly sorted, angular arkoses and lithic arenite. Includes the Arroyo del Agua and El
Cobre Canyon Formations (summarized from Lucas and Krainer, 2005).
lPm
Madera Group – Limestone with minor sandstone and conglomerate.
XYu Undivided Paleo and MesoProterozoic rocks – Quartzite, gneiss, and crystalline
intrusions. Includes Ortega quartzite and metavolcanic rocks.
STRUCTURE
The El Rito quadrangle encompasses an important structural transition in the Rio Grande rift
between the western San Luis Basin structural terrain (including the Tusas Mountains) and the
Abiquiu platform of the Española Basin. We use the term Abiquiu platform in a structural sense
to indicate the northwest arm of the Española Basin, located northwest of the Santa Clara and
Embudo fault systems, that approximates a faulted bench and where basin-fill is approximately 1
km-thick (Baldridge et al., 1994). Two west-down faults continue into this quadrangle from the
north. The more prominent of these faults is the Potrero fault (sensu Kelley, 1978), which may
have ~580 m of throw according to cross-section A-A’. Located on the eastern margin of
Arroyo Seco, this fault projects SSE towards CCC spring. Stratigraphic constraints suggest that
near CCC spring this fault intersects a major west-down, northeast-striking fault. The latter fault
continues NNE for 3 km past Las Placitas (where it locally includes left-strike-slip faults with a
minor component of reverse throw).
Faults in the southern part of the quadrangle strike northeast and generally exhibit normal
motion. They have variable dip and sense of throw in the southeastern part of the map. In the
southwestern part of the map, northeast-striking faults define a graben in the upper reaches of
Arroyo del Perro and Alamosa Canyon. The western faults in the southwestern quadrangle are
east-down and can be considered an extension of the Madera Cañon fault (which offsets the
Pliocene-age basalt on the eastern mesa of Sierra Negra). The eastern faults in the southwestern
quadrangle are either exposed or interpreted based on stratigraphic relations in lower Alamosa
Canyon and lower Arroyo del Perro. These eastern faults are west-down and do not appear to
continue for more than 1 km southwards beyond the southern quadrangle boundary. Strata west
of the southern part of the eastern faults dip 10-11° northeastwards into the graben, whereas
strata near the western faults generally dip ~10-12° to the southeast.
Page 23 of 31
Strata in the quadrangle generally strike northeast and dip 4 to12° to the southeast, paralleling the
strikes of most of the faults. An exception to this generalization is the prevalent northward strike
present near the Potrero fault. On the immediate hanging wall of the Potrero fault, strata strike
northwest, parallel to the fault, and dip steeply to the southwest. Another exception to the
northeast strikes is found in the highlands north of El Rito east of the Potrero fault. Here,
bedding strikes and dip directions are highly variable. The reason for this variability is
uncertain, but we suspect it relates to localized strain effects produced by numerous faults in the
area (many of which may not be directly recognized or mapped because of poor exposure). In
the southeastern quadrangle, east of El Rito Creek, dips decrease up-section from 8-14° E in the
lower Plaza lithosome and interbedded sand intervals (Ttpl and Ttcfl) to 5-8° E in the upper unit
of the Plaza lithosome and overlying Chama-El Rito Member (Ttpu and Ttc).
In the discussion below, we argue for middle Miocene activity along faults in this quadrangle
and related tilting. This activity may represent the culmination of fault activity in this part of the
basin. Throw rates here appear to decrease in the late Miocene and Pliocene as throw rates
increase along the Santa Clara fault in the center of the basin (as well as other faults belonging to
the Embudo and Pajarito fault systems; Baldridge et al., 1994; Koning et al., 2007a and 2007c).
It is important to note that fault activity in the northwestern part of the basin continued into the
Pliocene, at least locally and albeit at probable lower rates. For example, Pliocene activity along
the Madera Cañon fault is supported by the 11 m vertical offset (down-to-the-east) of the basalt
capping the eastern mesa of Sierra Negra (age of 5.56±0.12 Ma, Maldonaldo and Miggins,
2007).
RIFT-RELATED GEOLOGIC HISTORY AND
EVIDENCE FOR MIDDLE MIOCENE FAULTING
Here, we summarize the depositional history of the Rio Grande rift basin-fill in the map area.
The Ritito Conglomerate may represent the lowest rift-related strata in the study area, although
its poorly constrained age makes this inference uncertain. Deposition of the Abiquiu Formation
and Cordito Member of the Los Pinos Formation was made possible by a likely combination of
the following processes: 1) erosion and transport of detritus from the Latir volcanic field (both
during and following explosive eruptions), and 2) subsidence of the basin caused by Rio Grande
rift extension and volcanism-related isostatic subsidence and adjustments of stream profiles
(Smith et al., 2002). Progradation of the gravelly Cordito Member over the Abiquiu Formation
occurred in the northern map area ca. 20-21 Ma. The cause of this progradation is uncertain, but
perhaps is related to increases in discharge (due to possible climatic changes) or increases in
slope due to possible tectonic factors. The development of a major dacitic-andesitic eruptive
center in the south-central San Luis basin (i.e., the Plaza volcanic center of Ingersoll et al., 1990)
provided a source for the distinctive, porphyritic, dacite-andesite gravel of the Plaza lithosome
(Tesuque Formation). Dating of this gravel in the Tesuque Formation by Ekas et al. (1984)
indicates that this eruptive center is about 22-23 Ma (Ingersoll et al., 1990). The deposition of
the Plaza lithosome probably commenced a short time after the development of this volcano (ca.
20 Ma).
Page 24 of 31
Paleoflow data in the lower and middle Plaza lithosome, in addition to the Abiquiu and Los
Pinos Formations, indicate a general southwest flow direction (Figure 5), consistent with that
interpreted by May (1984). However, paleoflow data for the upper unit of the Plaza lithosome
(Ttpu) and overlying Ttc near the eastern quadrangle boundary show a general southeast flow
direction (Figure 5). This change in flow direction was accompanied by a significant change in
the gravel assemblage. In particular, the proportion of felsic gravel clasts (i.e., rhyolite, tuff, and
welded tuff) increased from less than 15% in the lower-middle lithosome P to 15-50% in the
upper lithosome P unit. Many of these felsic clasts are similar to those observed in the
underlying Abiquiu and Los Pinos Formations, although Amalia Tuff gravel seems slightly more
abundant in unit Ttpu than in the Abiquiu or Los Pinos Formations mapped on this quadrangle.
Basalt and orangish granite clasts also increased slightly (to 1-3% each); the basalt clasts are
similar to late Oligocene-early Miocene basalt flows mapped in the Valle Grande Peak
quadrangle and near Petaca in the La Madera quadrangle to the northeast. Both basalt and
orange granite was observed in the Abiquiu and Los Pinos Formations. However, intermediate
intrusive clasts (e.g., graniodiorite, quartz diorite, and tonalite), often having a slightly greenish
weathering rind and comprising 1-5% of the gravel fraction, were not seen in underlying basinfill strata (at least not on this quadrangle nor the Valle Grande Peak quadrangle to the north).
We interpret the increase in clast diversity after 15 Ma (the maximum age of the Ttpu unit) to an
increase in fault-related tilting and uplift in the southern Tusas Mountains. This increase led to
erosion of the Abiquiu and Los Pinos Formations in the southern Tusas Mountains, and the
detritus was recycled into Ttpu and overlying Ttc in the Tesuque Formation to the south. We
cannot explain the presence of the intermediate intrusive clasts by simple erosion of the Abiquiu
and Los Pinos Formations, since this clast has not been observed in either unit. Very brief
reconnaissance of mapped granodiorite in the Burnt Mountain and Mule Canyon quadrangles
(Manley and Wobus, 1982; Wobus and Manley, 1982) did not yield a satisfactory match with the
intermediate clasts in the Ttpu and overlying Ttc units. Another possible way to account for this
clast is by erosion of an older gravel located north of the Cordito Member of the Los Pinos
Formation. This conjectural gravel may have possibly been originally derived from the Sangre
de Cristo Mountains north of the Latir field, and later eroded by middle Miocene streams
developing in the Tusas Mountains. More work is needed to ascertain the provenance of the
intermediate gravel in the Ttpu and overlying Ttc units of the Tesuque Formation.
The change in paleoflow direction from southwest to southeast beginning ca. 15 Ma (the
maximum age of the Ttpu unit) may be explained by an increase in eastward tilting rates of the
basin floor in the eastern part of the quadrangle. That the basin-floor was actively tilting during
middle Miocene time is supported by a general up-section decrease in stratal dips east of El Rito
Creek. This tilting probably occurred due to subsidence along the Ojo Caliente fault, located 2-3
km to the east in the adjacent Ojo Caliente quadrangle, because it is the closest major structure
and bedding strikes in the eastern El Rito quadrangle parallel this fault (in contrast to the
southern Embudo fault system near the Picuris Mountains, which strikes more easterly at ~045°).
One must also consider motion along other faults in the Abiquiu platform when interpreting a
tectonic cause for this change in paleoflow. For example, if the east-down, western rift margin
faults near Abiquiu (discussed in Baldridge et al., 1994) also experienced a post-15 Ma increase
in throw rates, the streams may not change direction because of the presumed steepening of slope
Page 25 of 31
that would be created towards the hanging wall of the western margin faults. It is probably best
to relate tectonic activity in a relative sense. Accordingly, the southeastward change in
paleoflow direction is most likely caused by an increase in throw rate along the Ojo Caliente
fault relative to the western margin faults by Abiquiu.
During deposition of the Chama-El Rito Member subsequent to deposition of Ttpu, eolian sand
of the Ojo Caliente Sandstone (Tto) was occasionally laid down and preserved in the western
part of the quadrangle (Figure 4) between fluvial sediment of unit Ttc. Such tongues of Ojo
Caliente sand within the Chama-El Rito Member have been observed elsewhere in the Abiquiu
platform, most notably on the flanks of Lobato Mesa (in upper Cañon de la Madera, Daniel
Koning, unpublished data). It is possible these tongues of Ojo Caliente Sandstone represent
outliers of a larger body of eolian sediment to the southwest (i.e., up-wind) of the quadrangle.
This hypothesized eolian dune field to the southwest would have expanded northeastwards in a
dramatic fashion after 13.5 Ma, creating the main Ojo Caliente Sandstone deposit on this
quadrangle, whereas prior to this time its progradations were minor and short-lived.
HYDROGEOLOGIC INFERENCES
Although we did not directly measure the permeability of the map units, it is reasonable that
stratigraphically higher Santa Fe Group strata -- such as the Ojo Caliente Sandstone (Tto),
Chama-El Rito Member plus its fine sand-dominated intervals (Ttc and Ttcf), and the upper unit
of the Plaza lithosome (Ttpu) -- may possess somewhat higher permeability because of their
slightly coarser gross texture, less cementation, less consolidation, and general lack of a
tuffaceous matrix in their coarse channel-fill deposits. In contrast, underlying units typically are
better consolidated, show stronger cementation (although highly variable) by calcium carbonate
and silica, and seem to have a higher amount of reworked tuff (orangish clay) in the matrix of
their channel-fills. These lower units include the lower and middle Plaza lithosomes and
interbedded sands (Ttpm, Ttpl, Ttmpl, and Ttcfl), the Cordito Member of the Los Pinos
Formation (Tlpc), and the Abiquiu Formation (Ta). Relatively poor well yield in the Las
Placitas well was attributed to the presence of a tuffaceous clay matrix in what they called the
Los Pinos Formation (Geohydrology Associates, Inc., 1979), which likely also includes the
Abiquiu Formation. This clayey matrix may be responsible for the relatively low hydraulic
conductivity value (0.09-0.3 ft/day) calculated for the Los Pinos-Abiquiu Formations from a
pumping test in this well, (Koning et al., 2006). Consequently, units Tto, Ttc, Ttcf, and Ttpu
may produce slightly higher ground water yields than older basin-fill units. It is important to
note that cementation and clay cores associated with faults in basin-fill strata of the Santa Fe
Group may produce barrier effects, and where present may significantly reduce well yields, even
in the Ojo Caliente Sandstone (Koning et al., 2007d). Although the Potrero fault itself may act
as a barrier to groundwater flow, tectonic movement along this fault has allowed preservation of
stratigraphically higher, probably slightly more permeable strata on its west side. Wells drilled
west of this fault may therefore produce slightly higher yields compared to wells east of the fault,
unless local cementation and barrier effects from buried faults are present. Lastly, compilation
of pump-test data in the central and northern Espanola Basin indicates that the lithostratigraphic
unit possessing the highest hydraulically conductivity values there is Quaternary alluvium
Page 26 of 31
(Koning et al., 2007d). Two wells compiled in Koning et al. (2006) from this quadrangle that
were partially or totally completed in Quaternary alluvium produced higher yields and calculated
hydraulic conductivity (0.3-3.0 ft/day) than the aforementioned Las Placitas well (0.09-0.3
ft/day) in the Los Pinos-Abiquiu Formations. On this quadrangle, gravelly Quaternary alluvium
appears to be ~10 m thick adjacent to El Rito Creek and where present provides a reasonable
target aquifer.
ACKNOWLEDGMENTS
The U.S. Geological Survey allowed us to use their photogrammetric laboratory, which
greatly improved the precision of the mapping. As usual, Jim Messerich of the U.S. Geological
Survey proved his adeptness in setting photogrammetric models on the PG-2 plotter.
Conversations with local residents of El Rito concerning hydrologic issues – including Nelson
and Esperanza Gonzales and Juan Garcia – were helpful and appreciated. We appreciate Lynn
Heizler’s expertise in analyzing the basalt samples from Sierra Negra (Appendix 1).
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quadrangle, Rio Arriba county, New Mexico: New Mexico Bureau of Geology and Mineral
Resources, Open-file Geologic Map OF-GM-89, scale 1:24,000.
Koning, D.J., Skotnicki, S., Moore, J., and Kelley, S., 2005a, revised Nov-2007, Geologic map
of the Chili 7.5-minute quadrangle, Rio Arriba county, New Mexico: New Mexico Bureau of
Geology and Mineral Resources, Open-file Geologic Map OF-GM-81, scale 1:24,000.
Koning, D. J.; Karlstrom, K. E.; May, J.; Skotnicki, S. J.; Horning, R.; Newell, D.; and
Muehlberger, W. R., 2005b, Preliminary geologic map of the Ojo Caliente 7.5 minute
quadrangle, Rio Arriba and Taos counties, New Mexico: New Mexico Bureau of Geology,
Open-file Geologic Map-101, Scale 1:24,000.
Koning, Daniel J., Connell, Sean D., Morgan, Gary S., Peters, Lisa, and McIntosh, William C.,
2005c, Stratigraphy and depositional trends in the Santa Fe Group near Española, northcentral New Mexico: tectonic and climatic implications: New Mexico Geological Society,
56th Field Conference Guidebook, Geology of the Chama Basin, 2005, p. 237-257.
Koning, D.J., Hallett, B., and Aby, S., June-2006, Hydrogeologic and subsurface sedimentologic,
structural, and stratigraphic studies in the Tularosa and Española Basins, New Mexico:
unpublished report for the New Mexico Office of the State Engineer, 37 p., 31 figs, and 7
tables.
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Koning, D.J., Kelley, S.A., and Kempter, K.A., 2007a, Geologic structures near the boundary of
the Abiquiu embayment and Colorado Plateau – a long history of faulting [non-reviewed
mini-paper]: New Mexico Geological Society Guidebook, 58th Field Conference, Geology of
the Jemez Region II, .p. 43-46.
Koning, D. J., Karlstrom, K. E., Salem, A., Lombardi, C., 2007b, Preliminary geologic map of
the La Madera quadrangle, Rio Arriba County, New Mexico: New Mexico Bureau of
Geology, Open-fill Geologic Map-141, 1:24,000.
Koning, D.J., Connell, S.D., Slate, J.L., and Wan, E., 2007c, Stratigraphic constraints for
Miocene-age vertical motion along the Santa Clara fault, Española Basin, north-central New
Mexico: New Mexico Geological Society Guidebook, 58th Field Conference, Geology of the
Jemez Mountains Region II, p. 225-238.
Koning, D.J., Aby, S., and Finch, S., 2007d, Where is the water? – A preliminary assessment of
hydrogeologic characteristics of lithostratigraphic units near Española, north-central New
Mexico: New Mexio Geological Society Guidebook, 58th Field Conference, Geology of the
Jemez Mountains Region II, p. 475-484.
Manley, K., 1976, The Late Cenzoic History of the Española basin, New Mexico [Ph.D. thesis]:
University of Colorado, 1-171 pp.
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in the northwestern Española basin, and their relationships to the history of the Rio Grande
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and Tesuque formations near Ojo Caliente, northern Española Basin: New Mexico
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Abiquiu Formation in northern New Mexico: Albuquerque, N. Mex., University of New
Mexico, M.S. Thesis 147 p., 3 pl.
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Geology, vol. 26, no. 7, p. 933-965.
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Comments to map users
A geologic map displays information on the distribution, nature, orientation, and age
relationships of rock and deposits and the occurrence of structural features. Geologic and fault
contacts are irregular surfaces that form boundaries between different types or ages of units.
Data depicted on this geologic quadrangle map are based on reconnaissance field geologic
mapping, compilation of published and unpublished work, and photogeologic interpretation.
Locations of contacts are not surveyed, but are plotted by interpretation of the position of a given
contact onto a topographic base map; therefore, the accuracy of contact locations depends on the
scale of mapping and the interpretation of the geologist(s). Any enlargement of this map could
cause misunderstanding in the detail of mapping and may result in erroneous interpretations.
Site-specific conditions should be verified by detailed surface mapping or subsurface
exploration. Topographic and cultural changes associated with recent development may not be
shown.
The map has not been reviewed according to New Mexico Bureau of Mines and Mineral
Resources standards. Revision of the map is likely because of the on-going nature of work in the
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region. The contents of the report and map should not be considered final and complete until
reviewed and published by the New Mexico Bureau of Mines and Mineral Resources. The views
and conclusions contained in this document are those of the authors and should not be interpreted
as necessarily representing the official policies, either expressed or implied, of the State of New
Mexico, or the U.S. Government. Cross-sections are constructed based upon the interpretations
of the authors made from geologic mapping, and available geophysical (regional gravity and
aeromagnetic surveys), and subsurface (drillhole) data.
Cross-sections should be used as an aid to understanding the general geologic framework of the
map area, and not be the sole source of information for use in locating or designing wells,
buildings, roads, or other man-made structures.
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